On the entropic contribution to the redox energetics of SrFeO3-delta

The entropic contribution to the redox energetics of the grossly disordered high-temperature phase SrFeO3-delta is derived from the heat capacities of SrFeO2.500, SrFeO2.725 and SrFeO2.833 determined by adiabatic calorimetry. All three samples are structurally and magnetically ordered at low tempera tures and the calorimetric data allow deconvolution of the total heat capac ity and entropy into approximate contributions of vibrational, magnetic, el ectronic and configurational origin. Several thermal effects that are easil y accounted for in terms of known magnetic and structural transitions are o bserved and discussed. The configurational entropy of the disordered high-t emperature phase indicates substantial short-range-order on the oxygen sub- lattice even at high-temperatures. The standard entropy at 298 K as well as the total non-configurational entropy of SrFeO3-delta at high-temperatures varies approximately linearly with composition. The entropies of the follo wing oxidation reactions SrFeO2.50 (brownmillerite) + 1/6 O-2(g) = SrFeO2.833 (perovskite) SrFeO2.50 (perovskite) + 1/6 O-2(g) = SrFeO2.833 (perovskite) are deduced. The entropy of oxidation is within the uncertainty of measurem ent, independent of composition. On the other hand, the entropy of oxidatio n varies significantly with temperature. In general, the results substantia te the importance of the vibrational characteristics of reactant and produc t. While it is often assumed that the entropy of oxidation is close to -120 J K-1 (mol O-2)(-1), the present results indicate that the entropic contri bution to the redox energetics must be expected to vary to a large degree f rom one system to another.